1. Field of the Invention
Tape is the preferred medium for long term data (information) storage (archival storage) and data backup. This is due to low cost, high reliability and high storage capacity. Over the last 10-15 years, tape cassettes and tape cartridges have taken over for the single open tape reels used previously in the professional data market. These tape cassettes and cartridges simplify the handling of the tape, provide more protection and especially in the case of the so-called 3M cartridge, make it possible to design efficient tape drives at a very low cost.
2. Description of the Prior Art
There are many types of tape cassettes, but in reality only one type of well-known tape cartridge: the 3M cartridge originally developed by the 3M Company. The main difference between the tape cassettes and the tape cartridge is in the basic construction. A cassette typically contains either one or two tape reels in a protective housing white tape guides and the whole tape driving mechanism are outside (in the drive itself. The 3M cartridge contains two tape reels as well as tape guides and part of the tape driving mechanism. The 3M cartridge is therefore a more complex design than ordinary tape cassettes, but it simplifies the design of the drive mechanism.
For data backup and archival, four types of tape cassettes and the 3M cartridge are now dominating the market: The IBM 34 80/90 single reel 1/2" tape cassette; the DLT single reel 1/2" tape cassette; the 8 mm wide (video) tape cassette; the DAT (Digital Audio Tape) tape cassette and the 3M Data Cartridge. The 3M cartridge is very often also referred to as a QIC cartridge (Quarter Inch Cartridge). The name is derived from the original tape width (1/4") utilized in these cartridges.
For a long time, the QIC cartridge has been available in two sizes: The Standard cartridge suitable for use in tape drives having a 51/4" form factor, and a small cartridge, the Minicartridge, suitable for use in drives having a 31/2" form factor. The basic design oft cartridges is the same. FIG. 1 shows the basic design principles.
Some small details, which are not important for the invention described later, have been omitted.
The cartridge is built up on a solid baseplate of aluminum 100. All the necessary guiding pins and tape guides are mounted on this baseplate, as are the spindles for the two tape reels and the belt roller wheel. A plastic housing 101 is mounted on top of the baseplate and covers all the moving parts inside the cartridge.
The tape 104 is wound upon two tape reels 102 and 103 known as tape hubs. When the tape is running, the tape is either moving from reel 102 to reel 103 or vice versa. During its movement, the tape is guided to the front of the cartridge by a series of guiding pins tape guides 107. Upon insertion in a tape drive, a portion of the front end of the cartridge 106 acts as a door and opens to expose the tape to allow for contact with the drives recording head for read and write operations. During insertion in the tape drive, the door is opened and the cartridge then moved forward with the door open to bring the drive read/write head 115 in contact with the tape. FIG. 2A shows the cartridge from the top with the read/write head 115 and the drive wheel 117 engaged to the cartridge (as required for normal tape operation).
A drive puck wheel 105 (also referred to as a drive roller or a belt capstan roller) drives the tape indirectly through a belt 108 (hence the name "belt driven cartridge"). This belt proceeds outside the shaft of the drive puck 105, touches on the outside of the two tape reels 102 and 103 and goes around two comer rollers 109. As explained later, these corner rollers 109 are designed to provide a certain friction for the belt, thereby helping to build up and control the overall tape tension when the tape is running.
To sense the beginning and end of the tape, each tape end is equipped with some physical holes 110. These holes are sensed by the tape drive utilizing light. Light 112 is emitted from a lamp or similar light source in the drive to an internal mirror 111 through the top cover 101. The minor 111 reflects the light toward the backside of the tape. If a tape hole is present, some light 113 will pass through the hole (or holes) 110 and this is again sensed by a light sensing device in the tape drive.
For this system to work, the cartridge top housing or at least the area above and in front of the mirror, must be transparent. This is the way the tape hole sensing works for the large cartridge (51/4"). For the small cartridge, the system is reversed so that the light enter from the bottom through a hole in the baseplate. The principle of tape hole detection is the same.
The cartridge also contains an element 114 to physically allow or prevent writing on the tape. The basic principle is a small device which either is rotated (as for the large cartridge) or moved in parallel with the front (as for the small cartridge), to either expose or cover an opening in the front cover. The drive may then detect if this opening is exposed or not and thereby know if writing is allowed or not.
The baseplate 100 of the cartridge contains some small slots, one on each side to facilitate locking the cartridge in the tape drive. For clarity, these slots are not shown in FIG. 1.
The tape drives operating these cartridges are all equipped with a drive capstan motor 116 having a capstan wheel 117 which engages the drive puck wheel 105 when the cartridge has been properly inserted into the drive. This is shown FIG. 2A and 2B. Typically, the tape drive capstan motor 116 is designed so that it can tilt toward the cartridge drive puck 105. Normally, a spring 118 is used to press the capstan wheel against the drive puck 105. The tape drive motor 116 drives a capstan wheel 117 which drives the drive puck 105 in the cartridge which drives the belt 108. The belt 108 then moves the tape 104 by driving both tape reels 102 and 103. The tape direction is given by the rotational direction of the tape drive motor 116. The two corner rollers 109 are designed to provide a controlled friction force when the belt 108 is running to help build up the proper tape tension. Each of these rollers 109 has a built-in friction system. Tape tension is built up as the belt 108 drives the tape 104. It is important that the two corner rollers 109 be designed with enough internal friction to ensure rapid build up the tape tension. At the same time, the friction level should ideally be kept as low as possible to lower the driving force (the force required to drive the capstan wheel 117 in either direction) and the internal heat dissipation.
The cartridge also contains two guiding pins 119 and 120 which reduces the tension variations when the tape is moved from BOT (Beginning-Of-Tape) to EOT (End-Of Tape).
Due to the way this cartridge is designed, the tape tension can be built up only while the tape 104 is running. At standstill, tension is reduced to a very low level or zero. Therefore, it is important that proper tape tension can be built up rapidly every time the tape movement is started. This is also necessary when the tape changes direction. FIG. 3A shows the typical tension variations as the tape is run from BOT (Beginning OF Tape) to EOT (End OF Tape) or vice versa. At the beginning the tape tension is almost zero. As the belt starts moving the tape, tension is built up fairly rapidly typically to a level around 2 oz. (approx. 56 grams). As the tape movement gets closer to the EOT side, tension is typically increasing as indicated in the FIG. 3A. When the tape stops it will rapidly loose tension again. The tension profile is similar when running from the EOT side of the tape to the BOT side. As already mentioned, the two guiding pins 119 and 120 help reduce the total tension variation as the tape 104 is running from BOT to EOT, or vice versa.
FIG. 3B shows a typical tape tension when the tape is started from BOT then stopped in the middle of the tape and reversed back to BOT again. In this case, tension first builds up as shown in FIG. 3A. When the tape 104 is then stopped, it will immediately lose tension, so that the tension is very low when the reverse movement is started. Tension is then built up as shown. As the tape 104 gets back to BOT the tension is typically increasing above the average level as shown in FIG. 3B.
As can be seen from these figures, it takes some time for the tension to be built up every time the tape starts moving, or changes direction. Good tape tension is very critical in order to achieve a stable head-to-tape interface. For the typical tape drive designed for these cartridges, a tape tension of at least 1.4 oz. is required and it should ideally be between 2 and 3 oz. However, a high tape tension also normally means that the force to drive the cartridge (turning the cartridge drive puck wheel 105) must be higher, which again means that the drive and the cartridge require more power to operate. This power is partly dissipated in the motor 116, partly in the cartridge itself. Therefore, the interior of the cartridge and the cartridge components like the baseplate may get very hot during operations, especially at high tape speeds. This in turn may cause operational problems with the belt or the tape, or at least reduce the life time of the cartridge. Additionally, the heat dissipated in the cartridge requires the baseplate 100 to be made of metal in order to act as an effective heat sink. Still, even with such heat sinks, the base 100 can get very hot during operation, especially for the small Minicartridge.
As already mentioned, keeping proper tape tension when the tape is running is extremely important in order to ensure a good contact between the tape and the read/write head (recording head) in the drive. As new advanced tape drive systems are being developed, the linear bit density and the number of tracks is constantly increased. This makes head-to-tape contact more and more critical, which means that tape tension becomes more and more critical as well.
To ensure proper tape tension and tape movement, the interface between the driving belt 108 and the tape 104 is very critical. This is true both for the front side (magnetic media side) and the backside of the tape 104. If the belt surface 108 and the front side of the tape 104 are too smooth, the belt 108 is not able to run the tape 104 properly, especially at high speed and during fast accelerations/retardations, creating tape slippage and reduced tension. However, at the same time it is necessary to make the magnetic surface of the tape very smooth in order to ensure good head-to-tape contact and reliable read/write operations. Therefore, with a smooth tape front surface, the properties of the belt surface are extremely critical. It must be designed to ensure a reliable contact between the magnetic side of the tape surface and the belt during all normal operations and within the whole range of temperatures and humidity which the cartridge is designed to meet.
The design of the known cartridge is such that the belt 108 has to be designed narrower than the tape itself. The narrower the belt, the higher friction force (equal to tape tension) is required to ensure proper tape movement. However, the narrow high tension belt will leave a depression into the tape surface, creating problems with reading and writing the data on the tape 104 over time. This becomes especially important at very high track densities (high number of tracks), because each track is so narrow that even small irregularities may cause hard write or read errors.
The backside of the tape 104 is also very critical, it must be rough enough to ensure proper packing of the tape including air drain (escape) as the tape 104 is wound on to the tape reel. Since the tape reels 102 and 103 are actually driven by the belt 108 by pushing on the outside layer of the tape 104, it is very important that the backside is rough enough to ensure stable packing of the tape 104. If the backside of the tape 104 is too smooth, the tape 104 may be wound unevenly on the reel hub. This will very often result in tape jamming, as the tape 104 falls off the reel hubs 102 and 103, especially during fast acceleration and retardation and also during storage.
However, making the backside of the tape 104 too rough is also a problem, because the roughness of the backside to some extent will penetrate to the magnetic front side of the tape ("print through"). This will cause bad head to tape contacts and a large number of read and write errors. This print through problem increases as the manufacturer introduces thinner media to increase the total tape length. Therefore, with today's design of this tape cartridge, the manufacturers must balance the roughness of the back-coating, the belt properties and the thickness of the media to find the optimum compromise. Very often, the cartridge designer is forced to reduce the maximum speed or avoid very thin tape in order to find acceptable compromises. Hence, although high data transfer rate is also desired, some of the new data cartridges now introduced have a lower maximum speed limit that the older cartridge designs.
Another problem area with the current design is the use of the corner rollers 109 with their built-in friction force. As already mentioned, this friction force is required to build up the proper tension in the tape. These two friction areas exist in addition to three other major friction areas in the cartridge. The bearing between the cartridge drive puck wheel 105 and its internal axle (or spindle), and the bearings for the two tape reels 102 and 103. Additionally, extra friction is generated wherever the tape touches guiding pins or tape guides and in the interface between the belt 108 and the backside of the tape 104 and between the tape 104 and the head.
To overcome all this friction requires a lot of driving power. Typically as much as 7 watts are constantly being used to drive a Standard cartridge, and only slightly less with the Minicartridge. This generates heat which to a large extent dissipates inside the cartridge. As already mentioned, the interior of the cartridge and the cartridge components like the baseplate can be very hot, especially at high tape speeds and high external temperatures.
Without changing the basic principle of the cartridge as described above, several other tape cartridges have been introduced during the last years, however, all of them are based upon the basic principle of the 3M cartridge shown in FIG. 1. Sony has for some years offered a version with the trade name "Pegasus". FIG. 4 shows the basic form of this cartridge. The dotted lines show the position of the two tape reels and the internal tape path. Basically, it can be viewed as a cartridge having a front compatible with a Minicartridge so it fits 31/2" drive while the main portion of the cartridge is similar to a 51/4" cartridge (and thus remains outside the 31/2" drive). The main reason for producing the Pegasus cartridge is that it can contain far more tape than a minicartridge. The penalty is that most of the cartridge has to remain outside the tape drive all the time. The internal design principle in the Pegasus cartridge is the same as in the other two QIC cartridges.
Originally, the two types of QIC cartridges and the Pegasus cartridge contained tape which was 1/4" wide (hence the name QIC=Quarter Inch Cartridge). In 1992/93 Sony introduced another type of minicartridge with a wider tape; although the cartridge from the outside had the same dimensions as the original minicartridge. The wide tape minicartridge uses 8 mm wide tape to increase available tape area with approximately 27%. In principle, all available QIC cartridge types can be designed to use 8 mm wide tape without changing the outside cartridge dimensions.
Late 1994 and early 1995 two new versions of the QIC minicartridge were introduced to the market, both with the goal of increasing the available tape length further. The company 3M introduced a cartridge with the trade name "Travan". The basic design is shown in FIG. 5. The front of The Travan cartridge is the same as the Minicartridge, but the sides slope outwardly and the cartridges is deeper than a Minicartridge. Again, the doted lines show the approximate position of the tape reels and the internal tape path. This design allows for almost 100% more tape than in a comparable Minicartridge, while the size is such that the cartridge still can fit inside a 31/2" tape drive. The basic internal design of the Travan cartridge including the way it operates is identical to the Minicartridge and the Standard cartridge.
The other new cartridge was introduced in February 1995 by the companies Gigatek and Verbatim and named EX cartridge. This cartridge has the same width as the Minicartridge but is about twice as long (deep). The two internal tape reels are not mounted side by side, but offset so that one is farther from the front end of the cartridge than the other one. This allows for more tape on both reels. Actually, this cartridge contains 33% more tape than Travan cartridge. Except for the offsetting of the tape reels, the internal basic construction and operating principle is the same as for the normal Minicartridge. Since this cartridge has the same width and height as the normal Minicartridge it will fit into the opening of a standard 31/2" drive.
While it is possible to develop tape cartridges with radically different designs to overcome some of the problems mentioned above, such cartridges would also require new tape drive designs. Given the extremely high number of installed tape drives based upon the use of QIC cartridges (estimated to be more than 12 million drives by end of 1997), it is highly desirable to design a tape cartridge which overcome or reduces some of the most critical problems with the current cartridge type, while still being compatible with the current cartridge and therefore being usable in the drives designed for these cartridges.